1. Field of the Invention
The field of the present invention relates in general to semiconductor processing. More particularly, the field of the invention relates to a system and method for chemical vapor deposition (CVD) and thermal processing, such as epitaxial deposition.
2. Background
A variety of semiconductor processes require uniform thermal processing at high temperatures. Examples of such processes include rapid thermal anneal (RTA) and CVD. In many systems, high intensity lamps (usually tungsten-halogen lamps or arc lamps) are used to selectively heat a wafer within a cold wall furnace. Since the lamps have very low thermal mass, the wafer can be heated rapidly. Rapid wafer cooling is also easily achieved since the heat source may be turned off instantly without requiring a slow temperature ramp down. Lamp heating of the wafer minimizes the thermal mass effects of the process chamber and allows rapid real time control over the wafer temperature.
However, it is often difficult to control the temperature of the semiconductor substrate using only low thermal mass lamp heating. Temperature gradients during processing can lead to defects such as crystallographic slip and non-uniform deposition of materials onto the semiconductor substrate. Some reactors use a large thermal mass silicon carbide coated graphite susceptor to maintain temperature uniformity of the substrate during processing. The substrate is placed on the susceptor which acts as a thermal ballast to even out non-uniformities across the back side of the substrate. The substrate has a high thermal conductivity, absorbs radiant energy from the lamps, and conducts it laterally to maintain temperature uniformity. The susceptor is typically wider than the substrate which allows it to compensate for radiative heat loss at the edge of the substrate.
A large thermal mass susceptor, however, retains significant thermal energy and makes it difficult to rapidly adjust the temperature of the substrate. In addition, in CVD processes, material is often deposited on the susceptor as well as the substrate. As a result, the susceptors must be cleaned and replaced from time to time. In addition, in the event of damage to the susceptor surface, pin holes may form which expose the underlying graphite and contaminate substrates placed on the susceptor.
During CVD processes, such as epitaxial deposition, undesired deposits may form on the hot susceptor as well as on other components in the reactor chamber. The chamber walls are typically cooled to inhibit deposits. Nonetheless, deposits may accumulate over time and cleaning is often necessary. Typically, an HCL gas is used to etch the unwanted deposits from non-quartz surfaces during cleaning, but the etch is not always effective for deposits on quartz. Quartz components often must be removed from the reactor for cleaning. In addition, wear from processing and cleaning can cause components and materials in the chamber to flake or spall which may interfere with the quality of some processes.
What is desired is an improved apparatus and method for CVD and/or rapid thermal processing of a semiconductor substrate. Preferably, such a system and method would provide uniform substrate processing temperature without using a large thermal mass graphite susceptor. What is also desired is an improved apparatus and method for CVD processes, such as epitaxial deposition. Preferably, such a system and method would have reduced contamination, reduced deposition on chamber walls and components and reduced wear of components, while providing simplified cleaning.